The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine constructed for sucking evaporated fuel generated in a fuel tank into the intake side of the internal combustion engine so as to burn the evaporated fuel.
There has been an apparatus in which evaporated fuel generated in a fuel tank is stored in a canister, and the evaporated fuel stored in the canister is discharged together with air to a intake side of an internal combustion engine so as to be burnt, in which the quantity of discharged fuel, i.e., canister purge quantity is varied by a fixed value and concentration of the evaporated fuel sucked into the intake side of the internal combustion engine from the canister is detected by the variation of an air-fuel ratio at that time, thereby to correct an air-fuel learning value in accordance with this concentration (e.g., JP-A-2-130240).
In a conventional apparatus described above, however, there is a problem that correct air-fuel ratio learning cannot be sufficiently done since the concentration of the evaporated fuel is high at the initial stage of start of purging, and the variation of an air-fuel feedback value due to the influence by the evaporated fuel becomes larger than the variation of the air-fuel feedback value to be learnt, and the concentration of the evaporated fuel has not yet been detected correctly at the initial stage of the start of purging.